I defer to Pierino - the shoulder is just the meat from the area above the fore leg rather than the rear. The farm I usually purchase from packages lamb leg meat which definitely requires a liquid braise or stewing to become tender whereas their shoulder meat can be grilled and still be tender. Due to the shorter cook time in this recipe, I stuck with shoulder meat. However, this farm's leg meat is likely different than "leg of lamb", which is not something I generally cook. Maybe they cut from further down the leg or shank.